Industrial doors in which the door itself is made of pliable material such as fabric, are used in a variety of applications, typically for the purpose of separating areas within a building, or closing off building entries from the outside. Examples of such pliable doors are planar doors, overhead-storing doors and roll-up doors. Planar doors include frame members on which the fabric comprising the door is disposed. This plane of material is then movable between a doorway blocking position and a storage position, wherein the plane of material and associated frame members are disposed above the doorway. The frame typically includes extensions extending past either side of the door, and which are receivable within guide tracks to guide the door through its vertical movement. These extensions may include wheels or trolleys. An overhead-storing door is similar in that the fabric door is maintained on frame members and is movable between doorway blocking and storage positions. In this door, however, the storage position is overhead, as in a typical garage door. Accordingly, the guide members associated with such a door will curve between the vertical and horizontal. A typical roll-up door comprises a fabric curtain which is wound about a roller journalled for rotation above the doorway with which the roll-up door is associated. To close the door, the roller is rotated such that the curtain pays off of the roller to enclose the doorway. Of course, the door is opened by reversing the direction of the roller and rolling the fabric curtain onto the roller. Such roller doors are typically either powered opened and closed, or are powered open and allowed to fall closed by gravity. As the invention herein is envisioned for use primarily with roll-up doors, it will be described with reference thereto. However, the invention may also be used in combination with other such pliable doors.
When a roll-up door is placed over an exterior doorway of a building, provision must be made to prevent the fabric curtain from billowing due to wind being applied from the outside. Similarly, when the roll-up door is in place between different sections of a warehouse, there may be pressure differentials between these two sections, which may also cause billowing of the roll-up door if the door does not have provision to prevent this from happening. Such billowing may be problematic as it impedes door function and allows leakage around the door. To correct for this problem, roll-up doors typically include a rigid or semi-rigid bottom bar to help in providing what is generally referred to as "wind retention". The bottom bar typically extends across the leading width of the door, and also includes extensions which extend past either side of the door. These extensions typically engage side frames disposed on either side of the door and which run vertically along the side of the doorway. As the door moves between its open and closed positions, the bottom bar and its extensions move within a generally vertical plane since the extensions engage and are guided along or within the generally vertical side frames. With the leading edge of the door thus restrained within a vertical plane, movement of the fabric curtain of the door out of that vertical plane is largely avoided. However, the bottom bar only ensures that the leading edge of the door stays in the vertical plane, and strong gusts of wind or large pressure differentials between sections of a building may still allow the remainder of the curtain to billow either during the curtain's travel, or when it is fully closed.
To prevent this undesirable movement of the door, many prior art doors provide wind retention by use of a tensioning means to place a vertically disposed tension on the door to prevent it from billowing out of the vertical plane. One example of such a tensioning means is a heavy bottom bar. The weight of the heavy bottom bar may provide sufficient vertical tension to prevent undesirable billowing particularly (although not exclusively) in a gravity-fall type door. Alternatively, external means may be used to provide the necessary tension. For example, belting is often used for this purpose. Typically, one end of the belting is attached to the roller, and is wound and unwound from the roller in the opposite sense from the curtain. The belt is then passed through a pulley mounted near the bottom of the side frame. The other end of the belt is then attached to the extensions of the bottom bar. As the belt is wound and unwound from the roller in an opposite sense to the curtain, it exerts a downward pulling force on the bottom bar and the side frame inserts thus placing the necessary vertical tension on the door. Other particular arrangements for the belting besides that previously described are also used to achieve the same purpose.
A further exemplary means for exerting the necessary vertical tension on the door, at least in the closed position, is a system wherein the extensions of the bottom bar are latched in position when the door is in the closed position. In the case of the powered roll-up door, the motor is then reversed to exert the necessary vertical tension of the door to hold it taut.
While the variety of methods just described for wind retention are generally effective in preventing this problem, they are not without their own disadvantages. For example, obstacles in the path of travel of the bottom bar may be problematic. If an obstacle is in place in this position, and the door continues its downward movement, damage to either the door or the object could occur. Further, if the obstacle should be personnel, goods or equipment either damage to the door, goods or equipment or injury to the personnel could result. To avoid this problem, doors employing bottom bars typically also include some type of sensing mechanism for determining when an obstacle has been encountered. These sensors are coupled to the motor which drives the roller, and cause the door to be reversed upon encountering an obstacle. Such sensors, however, may be subject to malfunction, and add both cost and complexity to the door.